I realize that (well, assume really) you are trying to get a bit more life out of an older system with a few upgrades and spending as little cash as possible. But unfortunately, there comes a point where you need to decide whether it is worth it to spend money on upgrades, or would those dollars be better put towards a future replacement system. That is a decision that you will need to make on your own based on your personal circumstances.
When Tom's publishes their list of CPU's to buy, (eg. "Best Gaming CPUs For The Money: November 2013" http://www.tomshardware.com/reviews/gaming-cpu-review-overclock,3106.html) on the last page they have a grid of where each CPU stands.
They generally recommend only upgrading if you can move up at least two tiers on the chart.